The present invention concerns molded hook fasteners for use with hook and loop fasteners.
There are a variety of methods known to form hook materials for hook and loop fasteners. One solution is generally the use of continuous extrusion methods that simultaneously form the base layer and the hook elements, or precursors to the hook elements. With direct extrusion molding formation of the hook elements, see for example U.S. Pat. No. 5,315,740, the hook elements must continuously taper from the base layer to the hook tip to allow the hook elements to be pulled from the molding surface. This generally inherently limits the individual hooks to those capable of engaging only in a single direction while also limiting the strength of the engaging head portion of the hook element, as well as the density of the hook structures, which generally must point in the machine direction.
An alternative direct molding process is proposed, for example, in U.S. Pat. No. 4,894,060, which permits the formation of hook elements without some of these limitations. Instead of the hook elements being formed as a negative of a cavity on a molding surface, the basic hook cross-section is formed by a profiled extrusion die. The die simultaneously extrudes the film base layer and rib structures. The individual hook elements are then formed from the ribs by cutting the ribs transversely followed by stretching the extruded strip in the direction of the ribs. The base layer elongates but the cut rib sections remain substantially unchanged. This causes the individual cut sections of the ribs to separate each from the other in the direction of elongation forming discrete hook elements. Alternatively, using this same type extrusion process, sections of the rib structures can be milled out to form discrete hook elements. However, this approach is not commercially viable due to the speed of the milling operation. With this profile extrusion, the basic hook cross section or profile is only limited by the die shape and hooks can be formed that extend in two directions and have hook head portions that need not taper to allow extraction from a molding surface. This is extremely advantageous in providing higher performing and more functionably versatile hook structures.
The present invention provides a method for forming unitary polymeric structures comprising a polymeric base layer, and a multiplicity of spaced projections, projecting from at least one surface of the base layer. The method of the invention generally can be used to form upstanding projections, which may or may not be hook members that project upwardly from the surface of a polymeric film base layer. If the projections form hook members each projection comprises a stem portion attached at one end to the base layer, and a head portion at the end of the stem portion opposite the base layer. A head portion can also extend from a side of a stem portion. If a head portion is omitted entirely alternative projections can be formed which can be used for purposes other than as hook members. Multiple types of projections having different purposes can be produced on a single base layer as well. For hook members, a head portion preferably projects past the stem portion on at least one of two opposite sides. In the invention method, at least a portion of each projection precursor is heat treated so as to decrease the projection precursor thickness and thereby separating a projection from an adjacent projection. This heat treating also tends to reduce or eliminate molecular orientation in at least the heat treated portion of the projection in the machine direction.
The structured invention projections are preferably made by a novel adaptation of a known method of making hook fasteners as described, for example, in U.S. Pat. Nos. 3,266,113; 3,557,413; 4,001,366; 4,056,593; 4,189,809 and 4,894,060 or alternatively 6,209,177. The preferred method generally includes extruding a thermoplastic resin through a die plate, which die plate is shaped to form a base layer and spaced ridges or ribs projecting above a surface of the base layer. These ridges generally form the cross-section shapes of the desired projection to be produced. The die forms the spaced ridges and induces machine direction molecular orientation in the ridges by directing the molten polymer flow in the machine direction (the direction of polymer flow or extrusion). These ridges or ribs will also form the cross sectional shape of the projections as the ridges are formed by the die plate. The initial projection precursor thickness is formed by transversely cutting the ridges at spaced locations along their lengths to form discrete cut portions of the ridges. These cut portions are directly adjacent one another along the cut line so at this point they do not form discrete projections or form projections separated by only a minimal distance. In the past, longitudinal stretching of the base layer (in the direction of the ridges or the machine direction) would separate these cut portions of the ridges, which now separated cut portions would form spaced apart hook members based on the profile of the extruded ridge. However, in the present invention, cut rib or ridge portions are simply heat treated without stretching. The heat treatment results in shrinkage of at least an uppermost portion of the cut portion thickness by from 5 to 90 percent, preferably 30 to 90 percent. This causes a separation of the cut portion generally of at least 10 xcexcm, preferably at least 50 xcexcm thereby forming the discrete projection. The heat treatment can then continue to shrink more or all of the cut portion (e.g., at least a portion of the stem portion of the hook members or down as far as the cut of the cut portion). The resulting heat treated projections, preferably hooks, are preferably substantially upstanding and/or rigid.